Looking for a trigger

Two men in hazardous materials suits put numbers on the shattered glass and debris as they investigate the scene at the first bombing on Boylston Street in Boston on Tuesday near the finish line of the 2013 Boston Marathon, a day after two blasts killed three and injured more than 170 people. The bombs that ripped through the Boston Marathon crowd were fashioned out of ordinary kitchen pressure cookers, packed with nails and other lethal shrapnel, and hidden in duffel bags left on the ground, people close to the investigation said Tuesday.